Keywords

1 Introduction

Ventilation systems in mines are essential because of the recent increases in mining depth. Mining workplaces are where drilling, blasting, and diesel equipment generate a substantial amount of dust; hence, mine ventilation is more important in these areas. For example, methane and coal dust from the mining face of underground coal mines cause health problems (such as pneumoconiosis) in workers and are likely to cause explosion accidents. Ventilation ducts are the most common method of controlling dust in underground mines. In this study, the ventilation system in the workplace of mining faces was simulated with one blowing duct that provides fresh air from outside to the mine face, and one exhaust duct that draws in the polluted air from the working area and discharges it to the outside.

The purpose of this study is to present the primary data of the design of the duct ventilation system by numerical analysis of coal dust and methane distribution for mine ventilation systems of one duct (blowing only), and two ducts (blowing and exhaust), respectively. In particular, coal dust sizes of PM10 and PM2.5 (which cause pneumoconiosis for mine workers) were analyzed in this study to show the ventilation efficiency of fine dust in the working area.

Numerous studies have been carried out for experimental and numerical investigation of the ventilation efficiency with ducts in the mining face. Parra et al. [1] compared the field measurement data and the numerical analysis data to the MAA in an underground coal mine. They found that ventilation systems are efficient for the case of the exhaust duct located very close to the mining face and the blowing duct located relatively far from the mining face. Torano et al. [2] investigated the effect on workers by numerical analysis of airflow and dust concentration with field measurement data at the mining face with road headers using two ducts. Torano et al. [3] measured the concentration of methane in an underground coal mine and compared the results with numerical analysis. The measurement data of the mining field indicates there was a high methane concentration at the mining face. The methane concentration decreased to a minimum value at the ventilation duct and then increased with distance from the mining face. Thus, the authors reported that auxiliary ventilation systems are necessary for high ventilation efficiency. Diego et al. [4] developed a method to calculate air pressure loss by CFD modeling and performed 4D simulation for dispersion of pollutants in tunnels with a tunnel boring machine (TBM). Lihong et al. [5] showed CFD modeling using the experimental data at the mining face with a continuous miner in mine tunnel ventilated with a blowing curtain. Sasmito et al. [6] conducted a study on the design of a ventilation system using a blowing duct, exhaust duct, and brattice in an underground coal mine being mined with a room and pillar mining method. According to the numerical analysis results, the ventilation system using a brattice and exhaust system minimizes the appearance of a circular zone. Lu et al. [7] distributed methane and coal dust using blowing and exhaust ducts, and a continuous miner using room and pillar mining. The authors found that high airflow velocity and use of an exhaust duct generally lower the methane and dust concentration when continuous mining is in operation. Hasheminasab et al. [8] performed a numerical simulation to evaluate the distribution of methane concentration using a brattice and exhaust dust at an underground coal mining face. The results show that when using a brattice as a ventilation system, the use of an exhaust duct with a suction fan (auxiliary ventilation equipment) reduces methane concentration in the mine. Torno et al. [9] investigated the behavior of the carbon dioxide generated after a blast in a tunnel with a ventilation duct, and modeled the ventilation efficiency of the tunnel. The authors presented a new mathematical model (not a general mathematical model) that analyzed the statistical correlation with experimental data. Kurnia et al. [10] performed an analysis of the concentration and distribution of methane-based on the number and location of the methane sources in CFD modeling at the mining face. The simulation results show that methane dispersion is affected by the number and location of methane sources. The authors suggested that the method of reducing methane concentration is the focus of airflow at one point to reduce methane concentration. Park et al. [11] used the mean age of air (MAA) to indicate the freshness of the air by quantifying the air quality near the workplace in a mining tunnel. The results show an increase in MAA due to circular motion formed by the discharging of reflected fresh air from the mining face. Park et al. [12] performed a parameter evaluation to design a blowing duct to reduce MAA. CFD modeling was used to simulate the flow characteristics, vectors, and MAA depending on the location of the blowing duct to improve ventilation efficiency near the mining face. Therefore, the authors suggested that the best position of the blowing duct is the top of the center in the mining tunnel. Yi et al. [13] performed a parameter evaluation for optimal ventilation duct design. The optimal location and operation conditions of the ventilation duct are presented through MAA analysis by the location of the duct and the mass flow rate variable. The optimal location of the exhaust duct was suggested as the edge of the lower section of the mining face; the exhaust duct had no effect when the mass flow rate of the exhaust duct is less than 25% of the blowing duct mass flow rate.

In this study, the location of blowing and exhaust ducts were in accordance with Park et al. [12] and Yi et al. [13]. This study provides information on the distribution of coal dust and methane in the mining face using one duct or two ducts in the working area.

2 Mining Face and Workplace Arrangement

Figure 1 shows the geometry of mining workplaces with (a) one duct (blowing only) and (b) two ducts (blowing and exhaust). There are two types of workplace ventilation in mines: a single duct for blowing, and two ducts for blowing and exhaust. Park et al. [12] and Yi et al. [13] present the velocity distribution, mean age of air, and turbulent properties for the conditions of one and two ducts.

Fig. 1.
figure 1

Schematic diagram of geometry to compute flow distribution (a) one duct (blowing only), and (b) two ducts (blowing and exhaust)

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The mining tunnel features are the mining face, blowing duct, exhaust duct, outlet, roof, and bottom. The length and height of the mine tunnel were 36 and 2.9 m, respectively. The locations of the blowing and exhaust ducts are in accordance with Park et al. [12] and Yi et al. [13]. The blowing duct was located 0.14 m from the roof of the tunnel, and the exhaust duct was located 0.4 m from the bottom of the tunnel. The blowing and exhaust ducts had diameters of 0.6 and 0.3 m, respectively. The X-axis was along the length from the mining face to the outlet, the Y-axis along the height of the tunnel, and the-Z axis perpendicular to the X- and Y-axes.

3 Numerical Approach and Procedures

Numerical simulations using ANSYS CFX 19.1 were conducted to study the flow characteristics under the blowing duct and exhaust duct conditions in the mining tunnel. The software ICEM CFD 19.1 was used to generate the hexahedral grid for the simulation.

3.1 Independency of Numerical Grids

The structured meshes of the computational domain near the outlet are shown in Fig. 2. The hexahedral meshes were employed with the o-grid of ICEM-CFD 19.1 to improve the quality of the mesh near the air duct. A fine mesh was generated near the mining face and wall to resolve the high property gradient. In particular, the area where fresh air from the blowing duct hit the mining face is fine mesh because of its high-velocity gradient. Three different grids (0.5, 3.5, and 4.6 M) were checked for grid independency. The number of cells (3.5 M–4.1 M) was chosen to yield good results over a short time.

Fig. 2.
figure 2

Portion of the computed grid to compute the continuity and momentum equations: (a) one duct (blowing only), and (b) two ducts (blowing and exhaust)

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3.2 CFD Modeling

The purpose of this study was to characterize the flow and distribution of dust and methane using a blowing duct or blowing and exhaust ducts in a mine tunnel. The simulation fluid was air, coal dust, and methane. The coal dust diameters were set as 1 × 10−4 m, PM10 (10 × 10−6 m), and PM2.5 (2.5 × 10−6 m), respectively. In numerical analysis, particle movement can be modeled using the particle transport model of commercial code ANSYS CFX 19.1. The flow analysis of coal dust is solved by the Langrangian particle transport model of CFX. The particle transport model was used to analyze the dispersed phase (particles) in the continuous phase (air). The coupling between the two phases (air and particles) exists as a one-way, two-way, or full coupling; this study assumed a full coupling in which the dust and airflow affect each other [14]. The coal dust (PM10, PM2.5) was injected uniformly at the mining face. Methane was set to be released from the methane source for 5 min at the mining face.

The inlet boundary represents the velocity condition, which is 10 m/s, and the outlet boundary represents the condition of ambient pressure. The entire walls of the mine tunnel were set to be non-slip. The mass flow rate of the exhaust duct was set as 30% of the mass flow rate of the blowing duct, referring to Yi et al. [13]. Methane was released at a total flow rate of 0.05 m3/s at the mining face, in accordance with Torano et al. [3]. The total flow rate of dust was 0.0062 kg/s, in accordance with Lu et al. [7]. All numerical analyses were solved under steady and unsteady conditions. For methane distribution, the continuity and momentum equations were solved in the steady-state to reduce the computational time. Based on their solutions, the methane distribution equation was solved under the unsteady condition. For particle transport, all equations were solved under the unsteady condition.

3.3 Turbulence Modeling and Solution Convergence

The k-ε turbulence model was chosen to solve the flow distribution. The SST model was used to solve the transport equation because stable convergence of the solution was needed. The residual of the continuity and momentum equation was 1 × 10−6.

4 Numerically Predicted Flow Characteristic and Distribution of Coal Dust and Methane

4.1 Verification of Computation Model

To verify the numerical analysis in this study, the distribution of the velocity of x = 4 m was compared with the results of Kurnia et al. [10] from the mining face. Figure 3a shows the distribution of the velocity of x = 4 m using FLUENT software and the k-ε model in Kurnia et al. [10]. The result of the Kurnia et al. [10] simulation is based on the experimental data from Parra et al. [1]. The values of P (2.2, 1.8, 1.3, −3.2, and −4.4 m/s) are the experimentally measured data by Parra et al. [1]. Figure 3b shows the distribution of the velocity of x = 4 m using CFX software and the k-ε model in this study. The fresh air from the blowing duct has a negative value based on the X coordinate. The red area in Fig. 3a and the dark area in Fig. 3b indicate negative values. Fresh air was reflected from the mining face and discharged to the outlet, and thus it has a positive value. Although Kurnia et al. [10] used FLUENT software for numerical analysis, and CFX software was used in this study, the results in Fig. 3 can be assumed identical.

Fig. 3.
figure 3

Comparison of streamwise velocities in the (a) plane X = 4.0 from the mining face [Kurnia et al., 2014], and (b) same location as the plane in this study

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4.2 Distribution Characteristics of Airflow

Figure 4 shows a schematic diagram of planes in different locations in a mine tunnel. The planes are set as X = 3, 12, and 20 m to compute the air, coal dust, and methane flow quantities. X = 3 m is a plane near the working area.

Fig. 4.
figure 4

Schematic diagram of planes to compute profiles of airflow quantities

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Figure 5a shows the streamline flow of air from the mining face with a blowing duct velocity of 10 m/s. Fresh air from the blowing duct hit the mining face, and fresh air reflected from the mining face was discharged into the outlet. When fresh air was discharged into the outlet, vortical flow developed near the blowing duct. The vortical flow prevented fresh air and dust from discharging into the outlet, causing poor ventilation in the working area. Figure 5b shows the streamline flow of the mine tunnel with the blowing and exhaust ducts. The fresh air from the blowing duct was reflected by the mining face and flowed into the exhaust duct and outlet. Some air flows, generating vortical flow, but it can be seen that Fig. 5b is less vortical flow than Fig. 5a, because some air is discharged into the exhaust duct. Since mining works work in areas where the vortical flow exists, this study investigates the effect of vortical flow on the flow of coal dust and methane in the case of one duct or two ducts.

Fig. 5.
figure 5

Streamlines of airflow around the mining face with (a) one duct (blowing only), and (b) two ducts (blowing and exhaust)

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4.3 Distribution of Methane Using One or Two Ducts

Figure 6 shows the distribution of the methane mass fraction for time = 10, 20, 110, and 350 s in the case of one duct (blowing only). The distribution of the methane mass fraction is also symmetrical, and the streamline flow developed into asymmetrical flow. In the area of X = 3 m near the working area, the distribution of the methane mass fraction was the highest at time = 110 s. Methane was distributed at the bottom of the working area (X = 3 m) at time = 10 s, and with time, the methane spread with the airflow and was distributed throughout the mine tunnel. This phenomenon is shown in Fig. 7.

Fig. 6.
figure 6

Distribution of methane mass fraction at time = 10, 20, 110, and 350 s, at the X = 3, 12, and 20 planes for the case of one duct (blowing)

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Fig. 7.
figure 7

Distribution of methane mass fraction at time = 10, 20, 110, and 350 s at the Z = 0 plane for the case of one duct (blowing)

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Figure 7 shows the distribution of methane mass fraction at the Z = 0 plane of the on duct. The methane was initially distributed on the bottom, and then throughout the mine tunnel. The methane mass fraction is highest in areas where vortical flow exists. This explains how methane is stagnant by vortical flows. The methane mass fraction in the mine tunnel was the highest in all sections at time = 110 s and reduced at time = 350 s. After time = 350 s, the methane mass fraction near the mining face where working is done is low, with most methane are discharged to the outlet.

Figure 8 shows the distribution of the methane mass fraction for the case of two ducts (blowing and exhaust). As in the case of one duct, methane was distributed on the bottom at time = 10 s. The methane mass fraction of the two ducts was also the highest at time = 110 s. However, unlike the case of one duct, the methane was partially concentrated in the mine tunnel.

Fig. 8.
figure 8

Methane distribution at time = 10, 20, 110, and 350 s at the X = 3, 12, and 20 planes for the case of two ducts (blowing and exhaust)

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Figure 9 presents the distribution of the methane mass fraction at time = 10, 20, 110, and 350 s for the Z = 0, and 1.55 plane. The methane around the exhaust duct was discharged to the outlet.

Fig. 9.
figure 9

Methane distribution at time = 10, 20, 110, and 350 s for the Z = 0, and 1.55 plane for the case of two ducts (blowing and exhaust)

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4.4 Distribution of Coal Dust Using One or Two Ducts

The particle transport model was used to resolve the distribution of coal dust generated during coal mining. In particular, PM10 and PM2.5 coal dust are dangerous substances that can cause pneumoconiosis in workers. Thus, flow simulations of PM10 and PM2.5 coal dust are very important in terms of the working environment in underground coal mines. Figure 10 shows the streamline of coal dust for the case of one duct. The black, red, and blue lines are coal dust, PM10, and PM2.5, respectively. Coal dust moved into the bottom of the mine face under the influence of the flow of fresh air discharged from the blowing duct. PM10 and PM 2.5 flowed to the bottom of the mine face, with some scattering throughout the mine tunnel. Because PM10 and PM 2.5 had small diameters, it was judged to be sporadic scattering without the effect of airflow. Coal dust that moved to the bottom of the mine face flowed along the bottom of the mine tunnel to the outlet. PM2.5 flowed to the outlet because of its small diameter, and PM10 fell to the bottom at the midpoint of the mine tunnel. PM10 and PM 2.5 flowed to the vortical flow with the air. Vortical flow causes PM10 and PM2.5 to stagnate, which affects the health of mining workers. Therefore, ventilation systems of mine tunnels are essential to minimize vortical flow around the working area.

Fig. 10.
figure 10

Streamlines of coal dust flow around the mining face for the case of one duct (blowing only)

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Figure 11 shows the averaged volume fraction of coal dust for the case of one duct. As mentioned, the averaged volume fraction of coal dust (1 × 10−4 m) presents as zero in the contour because the coal dust flowed along the bottom of the mine tunnel.

Fig. 11.
figure 11

Distribution of averaged volume fraction of dust at the planes of X = 3, 12, and 20 for the case of one duct (blowing)

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Figure 12 shows the streamline of coal dust for the case of two ducts, similar to that for the case of one duct. Coal dust flowed to the bottom, and PM10 and PM2.5 flowed throughout the mine tunnel with the airflow. However, the case of the two ducts had less coal dust in the working area than that of the one duct, as some PM10 and PM2.5 were discharged by the exhaust duct to the outlet. As shown in Fig. 13, the averaged volume fraction of the coal dust was lower than that of the one duct. This seems to have caused coal dust to be discharged to the outlet by the exhaust duct.

Fig. 12.
figure 12

Streamlines of coal dust flow around the mining face for the case of two ducts (blowing and exhaust)

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Fig. 13.
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Distribution of averaged volume fraction of dust at the X = 3, 12, and 20 planes for the case of two ducts (blowing and exhaust)

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Figures 14 and 15 show the averaged volume fraction of coal dust in the iso-surface for the case of one and two ducts, respectively. Coal dust flowed along the bottom of the mine tunnel, and PM10 and PM2.5 were distributed throughout the mine tunnel with the airflow. All coal dust was discharged after time = 350 s. The case of the two ducts had a lower average volume fraction of coal dust than the case of the one duct. The results show that the ventilation system in the mine tunnel was effective with the blowing duct that blew fresh air into the working area, and the exhaust duct that discharged polluted air.

Fig. 14.
figure 14

Distribution of averaged volume fraction of dust for time = 10, 20, 110, and 350 s at the iso-surface for the case of one duct (blowing)

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Fig. 15.
figure 15

Distribution of averaged volume fraction of dust for time = 10, 20, 110, and 350 s at the iso-surface for the case of two ducts (blowing and exhaust)

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The future study plan is to investigate the distribution of coal dust and methane for increasing mass flow rates of the exhaust duct.

5 Summary and Conclusions

This research presents an analysis of three-dimensional fluid flow and particle transport for mining workplaces that have a blowing duct only or blowing and exhaust ducts. These results aid in understanding the characteristics of fluid flow and particle transport around the mining face in mines. Moreover, they facilitate more efficient duct design by the ventilation engineers. The length and height of the mining gallery were 36 and 2.9 m, respectively. Two types of ventilation ducts were selected: one duct for blowing only, and two ducts for blowing and exhaust. The commercial code ANSYS CFX 19.0 was chosen to solve continuity, momentum, and particle transport. ICEM-CFD 19.0 was employed to create hexahedral grids for better solutions. In summary, the results show airflow streamlines, methane distribution, dust streamlines, dust volume fraction for the cases of blowing only, and blowing and exhaust.

In the case of methane distribution, the highest mass fraction exists under the discharge plane of a blowing duct. Because blowing flow generates circular motions. In the circumstance of the blowing and exhaust duct, the highest mass fraction exists near the suction plane of an exhaust duct. Because methane distribution is identical to airflow, the position of an exhaust duct is very important. It can control the highest point of mass fraction for methane. With or without the exhaust duct, the heavy dust shows the same results. However, the fine dust of PM 2.5 or PM 10 can be easily affected by airflow. The volume fraction of fine dust is lower in the circumstance of an exhaust duct. It means exhaust ducts are needed when fine dust in the workplace of mining face in a mine are dominant.